As altitude increases in Earth's atmosphere, both air pressure and the amount of oxygen decrease.
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Air Pressure: Atmospheric pressure decreases with altitude. This is because air is composed of gases that are pulled towards the Earth's surface by gravity. As you go higher, there is less air above you, and thus the weight of the air (which contributes to pressure) decreases. The decrease is generally exponential; for example, at sea level, the average atmospheric pressure is about 1013.25 hPa (hectopascals), but it drops to about 700 hPa at 3,000 meters (9,843 feet) and continues to decrease at higher elevations.
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Oxygen Concentration: The concentration of oxygen (as a percentage of the total atmosphere) remains relatively constant at approximately 21% up to around 10,000 meters (32,800 feet) or so. However, since the overall air pressure is lower at higher altitudes, the partial pressure of oxygen also decreases. This means that while the proportion of oxygen remains the same, the amount of oxygen available for breathing decreases significantly, which can lead to hypoxia (oxygen deficiency) in people who ascend rapidly to high altitudes without acclimatization.
Overall, as altitude increases, the air becomes less dense, leading to lower pressure and a lower availability of oxygen for respiration.